The same principles which allow aircraft to fly are similar in racecar aerodynamics, but the main focus is to produce downforce instead of lift (know as negative lift).
Depending on the exact requirement for the application, the set up can be modified to suit top speed (lower drag levels) or high down force (higher drag levels), while providing more grip for the corners by pushing down on the tyres/ tires. The ideal set up is normally to get the maximum amount of downforce, for the smallest amount of drag generated. Gear ratios, track configurations and regulations all have an effect on the overall package, which must be viewed as a whole.
Another important factor is aerodynamic balance, which will have an effect on the understeer versus oversteer handling characteristics of the car- especially at speed as most aerodynamic devices start to work exponentially with speed.
Downforce Set Ups for Racetracks
Below are some general guidelines, of what downforce set ups will be the most advantage for different tracks layouts:
1. Racetracks with long high speed straights and the majority of slower speed corners, favour low downforce/ drag set ups.
2. Racetracks with fast sweepers and short straights, favour a more balanced downforce/ drag set up.
3. Racetracks like street circuits, with short straights and a mixture of fast and slow corners, favour a high downforce/ drag set up.
(Bear in mind that we are speaking about a general rule of thumb, with a well balanced and correctly set-up racing car).
Downforce Generating Devices
Most race cars aerodynamics will have a selection of different settings for the various aerodynamic aids (front wing, rear wing, and diffuser for example) to get the optimum set up for the highest lap times, depending on the requirements. While this might not always be the case with normal productions cars, some vehicles might be set up to produce real down force (Ford Escort Cosworth for example) while others are more geared towards fuel efficiency and visual appealing looks, with the intention to stop lift and reduce flow separation and not to create downforce.
The 3 main areas of the racing car downforce generating devices, which can be developed are (the overall body is important but is normally predetermined at point of initial design):
Bernoulli Effect
One of the main physical forces involved in down force generation is called the Bernoulli Effect, fundamentally meaning that if a fluid (gas or liquid) flows around an object at different speeds. The slower moving fluid will exert more pressure than the faster moving fluid on an object.
The object will then be forced toward the faster moving fluid, creating negative or positive lift.
Slower moving airflow = Higher pressure.
Faster moving airflow = Lower pressure.
With racecar aerodynamics we need to force more high speed (low pressure) air to go under the aerodynamic devices creating negative lift (downforce). The harder and faster you drive, the more downforce is produced and the more grip will be available to the tyres/ tires via the suspension and chassis, effectively sucking the car to the track with ever increasing speed. Downforce is produced at the square of velocity travelled- we have to bear in mind that every action has a reaction (Newtons Third Law), interestingly enough, drag is also produced at the square of velocity travelled.
More downforce results in higher grip levels for the tyres and more traction, especially going through the corners, braking and accelerating all enhanced. Tyres/ tires and available grip levels have their limits and any additional downforce produced past this optimum yield will be wasted and create unnecessary drag penalties. So it is critical to have the right tyre/ tire to utilise available grip, wear rates will also factor into this equation as the race progresses.
A F1 car can produce enough downforce to drive upside down (three times the car's weight in certain configurations). It can produce more downforce than the weight of the racecar and this force is the square of the object velocity (double the speed and you get quadruple downforce and drag levels).
While this downforce generation is desired if you going from 200 Mph straight into a sweeping corner or hairpin. A dragster or race car focused on top speed will be more interested in a low drag set up. This extra drag will hinder top speed and more engine power will be required to propel the car forward. It is a balancing act for top speed and downforce levels, hence the reason most competitive race cars will have adjustable aerodynamic aids to suit the best downforce levels for a given race track.
A Indy car for example would probably be generally set up to have a greater top speed then a F1 car as a comparison, as the F1 car will require greater levels of grip and downforce for the corners, especially with its rapid directional changes to reduce lap times. Otherwise high downforce levels will compromise the rest of the course, for high top speed on the straights.
It all comes down to different set ups for different race courses. Like a lot of components on race cars, it is possible to adjust the aerodynamic levels of the required downforce to yield greater top speeds depending on the circuit. The introduction of dynamic downforce devices like the “Drag Reduction System” will somewhat change this situation. From the 2011 F1 season, it allowed for less downforce generation to aid overtaking on the straights, by changing the rear wings angle of attack wing profile.
Here is an interesting fact: the average atmospheric pressure at sea level is 14.7 psi on all sides of an object, even on our own bodies. By reducing the pressure under the bonnet/hood to 14.5 psi, over an area of just a square yard, we would generate about 260 pounds of downforce (0.20 psi difference in pressure) x 362 (number of square inches in a square yard)= 259.2 pounds of usable downforce. So these small little adjustments can yield impressive results.
Racing cars and aerodynamics have come a long way in terms of development since basic ground effects where first applied to cars. In the early days of 1967, cars like the Lotus 49 and Lotus 79 for example, initially had huge rear wings, which were mounted on the rear suspension mounts. A little later they even introduced cable operated wings to reduce the angle of attack on the straights to increase top speed, the same as DRS in F1 2011 season effectively.
Due to too many accidents with these high mounted wings and the lack of technology in materials, resulting in fatal fatalities for both spectator and driver alike. An all out banned was introduced, but after negotiations engineers and designers started to look at other areas of the car to create even more down force levels with the now restricted wing regulations. Most of the principals applied where taken from aeronautical design and modified for use in Motorsport, with both failure and success depending on its application.
Race teams normally have huge budgets and dedicated engineers striving to continually create more and more effective designs, all the while battle to gain the upper hand on regulations. Sometimes we are only talking about 0.1 or 0.2 of a second difference on a total lap. But over the course of a Race, this can make the difference between first and second place. As we can see the introduction of ground effects and down force has made a big impact on the world of motorsport
You can see that there are a lot of benefits that can be made in terms of aerodynamics and manufactures/race teams spend huge amounts of money testing in wind tunnels to develop the most efficient aerodynamic designs. It is an ever increasing battle for more down force for as little drag as possible.
It is important to also reflect on the various aerodynamic advances over the last few decades, these have all been ways of conforming to the various Governing bodies rules and regulations (to the extreme in most cases). I suspect with unlimited scope, racecars pulling more than 6 g's in the corners are within technological realms of current technologies.
Huge amounts of potential downforce can be generated with a little thought and good design principles. This is an ever ongoing battle for engineers both in the world of production and motorsports cars. We have seen some weird and wonderful designs over the years and the future of all Innovations in car design will always continue to evolve.
There are lots of areas of nature in which engineers and designers' draw inspiration from, especially true for birds and aquatic mammals and fish. The shape of wings and fins hold millions of years of evolutionary engineering to overcome similar restraints, to the same laws of physics. I expect to see lots of continued development in the future emulating the natural world.
Wing Downforce Generation
Lets explore some of the areas of interest when generating downforce with an aerofoil or wing for Motorsport applications:
1. Angle of Attack
Angle of attack is the angle of the wing plane and the direction of motion. Increased angle of attack (wing angle) results in increased downforce and drag levels. Reducing the angle of attack, decreases the drag and downforce levels.
Bear in mind this is only in optimum operating angles, too much or too little angle of attack will result in wing stall.
Angle of Attack Versus Handling Balance
Angle of Attack | Handling Balance |
|---|---|
Increased Front Wing Angle of Attack | Decreased understeer |
Decreased Front Wing Angle of Attack | Increased understeer |
Increased Rear Wing Angle of Attack | Decreased oversteer |
Decreased Rear Wing Angle of Attack | Increased oversteer |
Aerodynamic Balance
What happens when we have different downforce levels compare to the front and rear of the vehicle, lets explore.
Downforce Balance | Handling Balance |
|---|---|
High Front to Rear Downforce | High speed oversteer |
High Rear to Front Downforce | High speed understeer |
2. Wing Aspect Ratio
This is the ratio of a wing span and chord. The higher the wings aspect ratio the greater the wing efficiency. It is also important to bear in mind that airflow ahead of the wing will also determine the wings´ potential efficiency of its full potential downforce levels.
You can have the best design wing in the world, but unless it has smooth clean airflow feeding it, this reduces its potential. A front wing with a lower angle of attack could produce more downforce than a rear wing with a higher angle of attack, purely because of the airflow. This problem has been one of the issues of overtaking a leading car- experienced in open wheel racing series like F1, drag reduction and other work arounds were deployed to negate this issue and to make racing exciting again.
3. Wing Centre of Lift
This is the centre part on a wing or aerodynamic generating device, where all the downforce acts upon. It has some similarity with the car´s
centre of gravity, in that it is a force which can be applied to the body and create a leverage effect, in some cases.
Centre of Lift, on a wing for example, is normally generated 1/3 of the way along the wing´s chord, from its leading edge. In the case of multiple element wing configurations, this centre of lift will move rearward.
It is important for wing supports or mounting points to be located as close as possible to the centre of lift, other wise downforce could act as a lever on the pivot point. This has effects on the wings angle of attack, under different downforce levels and can be viewed as dynamic in nature.
4. Wing Lift Generation
In order for a wing to generate negative lift (downforce), we need to ensure that when airflow reaches a wings leading edge, air is forced to travel a longer distance under the surface than on top of it.
This results in lower pressure being generated, as the underside of the wing increases the airflow.
5. Wing Lift to Drag Ratio
This is the ratio of negative lift (downforce) and drag, a wing will produce, at a given air speed and wing angle of attack. Logically you might image this would be represented by C1 (lift force) and Cd (coefficient of drag) figures. Due to wing aspect ratios and airflow conditions (laminar or turbulent), these figures may not be actual performance indicators in the real world and just dimensionless indicators.
This is one of the reasons that pure design is not based solely on CFD software, as wind tunnel and racetrack testing is also needed to confirm initial performance indications.
6. Wing Stall
If the wing have too much or too less angle of attack, the wing will stall, causing flow separation. Drag levels will dramatically increase as a result of either the angle of attack or the airflow ahead of the wing in motion.
One way in which designers have over come this is with multiple element wings, this allows far greater levels of angle of attack, why still keeping the airflow laminar and attached. Basically the high pressure air goes through the element slots speeding up the underside of the wing.
Is Wing Stall a Bad Thing for Aerodynamic Performance?
Yes if the whole rear wing stalls this is a big problem, but we have also seen the flexing rear wing slots and even F-Duct present in F1, these solutions effectively stalled just the wing element and not the main wing section. This results is a negative effect on the lift/ drag ratio, but a big reduction on overall drag. Resulting in a overall increase in top speed because of the rear wing reduction in overall drag.
At top speeds overall downforce is not required, as in the corners and braking zones.
This wing element stall could have an advantage at the front of the car also, because if this was applied to the front wing, overall downforce would be reduced, resulting in reduced drag and further increases top speed.
